1. Field of the Invention
The present invention relates to a cooling-control system for an ambient to be cooled, a method of controlling a cooling system, as well as a cooler, particularly making use or a compressor with variable capacity applied to cooling systems in general, this system and method enabling one to use conventional thermostats of the type that alter the conduction condition of a contact depending upon the minimum and maximum limits of temperature of the compartment or ambient to be cooled, permitting adjustment of the rotation or characteristics of the compressor, so as to maximize the performance of the cooling system.
The basic objective of a cooling system is to maintain a low temperature inside one (or more) compartment(s) or ambient(s) to be cooled, making use of devices that convey heat out of the latter to the external ambient, by resorting to measurements of temperature inside said compartment(s) or ambient(s) to control the devices responsible for conveying heat, trying to maintain the temperature within pre-established limits for the type of cooling system in question.
Depending upon the complexity of the cooling system and of the type of application, the temperature limits to be maintained are more restrict or not.
A usual way of conveying heat out of a cooling system to the external ambient is to use a hermitic compressor connected to a cooling closed circuit (or cooling circuit), through which a cooling fluid or gas circulates, this compressor having the function of causing the cooling gas to flow inside the cooling closed circuit, and is capable of imposing a determined difference in pressure between the points where evaporation and condensation of the cooling gas occurs, enabling the processes of conveying heat and creating a low temperature to take place.
Compressors are dimensioned to supply a cooling capacity higher than that required in a situation of normal operation, and critical situations are foreseen. Then some kind of modulation of the cooling capacity of this compressor is necessary to maintain the temperature inside the cabinet within acceptable limits.
2. Description of Related Art
The most common way of modulating the cooling capacity of a compressor is to turn it on and off, according to the evolution of the temperature inside the ambient to be cooled. In this case, one resorts to a thermostat that turns the compressor on when the temperature in the ambient to be cooled exceeds the pre-established limit, and turns it off when the temperature inside this ambient has reached a lower limit, also pre-established.
A known solution for this control device for controlling the cooling system is the combination of a bulb containing a fluid that expands with the temperature, installed so as to be exposed to the temperature inside the ambient to be cooled and mechanically connected to an electromechanical switch that is sensitive to that expansion and contraction of the fluid existing inside the bulb. It is capable of turning on and off the switch at predefined temperatures, according to its application. This switch interrupts the current supplied to the compressor, controlling its operation, maintaining the internal ambient of the cooling system within pre-established temperature limits.
This is further the most widely used type of thermostat, since it is simple, but it has the limitation of not permitting adjustment of the speed of a compressor of variable capacity, because it generates the command of opening and closing a contact responsible for interrupting the power fed to the compressor.
Another solution for controlling the cooling system is to use an electronic circuit capable of reading the temperature value inside the cooled ambient by means of a PTC-TYPE (Positive Temperature Coefficient) electronic temperature sensor, for example; or another one, comparing this temperature value read with predetermined references, generating a command signal to the circuit that manages the energy fed to the compressor, providing the correct modulation of the cooling capacity, so as to maintain the desired temperature inside the cooled ambient, be it by turning the compressor on or off, or by varying the supplied cooling capacity, in the case in which the compressor if of the variable capacity type. A limitation of this type of thermostat is the fact that it incorporates an additional cost for promoting the adjustment of speed of the compressor, requiring its correct adaptation for this function, by means of some capacity of logic processing and control algorithms that define the correct operation speed of the compressor, implemented in the thermostat circuit, separately from the control over the compressor.
Another solution for controlling the temperature in a cooled ambient is described in U.S. Pat. No. 4,850,198, which discloses a cooling system comprising compressor, condenser, expansion valve and evaporators, in addition to a control over the energization of the compressor. This control is effected by means of a microprocessor according to a readout of temperature from a thermostat determining energization or no energization of the compressor on the basis of predetermined maximum and minimum temperature limits. According to this system, control over the operation time of the compressor depending upon the temperature measured in the cooled ambient is foreseen.
One also knows from the prior art the solution presented in document WO 98/15790, in which the speed of the axle and, consequently, the cooling capacity of the compressor is adjusted by the controller, resorting to the information on opening and closing the contacts of a simple thermostat, of the type that promotes the opening and closing of the thermostats of a switch depending upon two temperature limits. This technique adjusts the compressor speed to each operation cycle, reducing the compressor speed ii each cycle, in predefined steps.
The limitation of this solution is that the most adequate operation condition for the compressor is sought step by step in each cycle, which makes the system slower and limits its benefits. It also has a limitation in the reaction time, when a substantial increment in cooling capacity is required along a cooling cycle, limiting the capacity of stabilizing the temperatures and limiting the response to the addition of thermal loads to the cooler.
Another solution known from the prior art is described in document U.S. Pat. No. 5,410,230, in which one proposes a control, by which the operation speed of the compressor is adjusted in response to the temperature and a determined point of the cooling system, requiring a temperature measurement circuit, with the consequent cost disadvantages.